This document discusses classes and objects in Python. It defines a Calculator class and demonstrates how to create class attributes, methods, and instances. It explains the __init__ method, self keyword, and how to access attributes and methods. It also covers data attributes versus class attributes, inheritance, method overriding, and calling parent methods. The document provides examples to illustrate these object-oriented programming concepts in Python.

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Classes and Objects
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
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Python Programming Essentials

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Contents
 Defining a class
 Class attributes
 Class methods
 Class instances
 __init__ method
 self keyword
 Accessing attributes and methods
 Deleting attributes
 Types of attributes
 Inheritance
 Method overriding
 Calling parent functions
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Defining a class
A class is a special data type which defines how
to build a certain kind of object
class className():
# statements
Use the class keyword to define a class
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Defining a class
class Calculator():
counter = 0
def __init__(self):
pass
def add(self):
pass
class keyword to
define a class
A class definition creates a class object from which
class instances may be created
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Class Attributes
class Calculator():
counter = 0
def __init__(self):
pass
def add(self):
pass
class attributes are
just like variables
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Class Methods
class Calculator():
counter = 0
def __init__(self):
pass
def add(self):
pass
class methods are
functions invoked on an
instance of the class
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Class Instances
calc = Calculator()
• In order to you use it we create an instance of
class
• Instances are objects created that use the class
definition
Just call the class definition like a function to create
a class instance
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__init__ method
• __init__ method is like an initialization
constructor
• When a class defines an __init__ ()
method, class instantiation automatically invokes
__init__() method for the newly created
class instance
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__init__ method (2)
class Calculator():
counter = 0
def __init__(self, x=0, y=0):
self.x = x
self.y = y
def add(self):
pass
calc = Calculator(10, 20)
print calc.x
print calc.y
Output:
10
20
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self keyword
• The first argument of every method is a reference
to the current instance of the class
• By convention, we name this argument self
• In __init__, self refers to the object currently
being created
• In other class methods, it refers to the instance
whose method was called
• Similar to the keyword this in Java or C++
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Accessing attributes and methods
Use the dot operator to access class attributes and
methods
calc = Calculator(10, 20)
print calc.x
print calc.y
print calc.counter
Output:
10
20
0
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Accessing attributes and methods (2)
class Calculator():
counter = 0
def __init__(self, x=0, y=0):
self.x = x
self.y = y
def add(self):
return self.x + self.y
calc = Calculator(10, 20)
print calc.add()
• Although you must specify self
explicitly when defining the
method, you don’t include it
when calling the method
• Python passes it for you
automatically
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Deleting Instances
• When you are done with an object , you don’t have
to delete or free it explicitly
• Python has automatic garbage collection
• Python will automatically detect when all references
to a piece of memory have gone out of scope.
Automatically frees the memory.
• Garbage collection works well, hence fewer memory
leaks
• There’s also no “destructor” method for classes.
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Attributes
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Two kinds of Attributes
1. class attributes
2. data attributes
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Data attributes
class Calculator():
counter = 0
def __init__(self, x=0, y=0):
self.x = x
self.y = y
def add(self):
return self.x + self.y
calc = Calculator(10, 20)
print calc.x # 10
print calc.y # 20
calc2 = Calculator(15, 35)
print calc2.x # 15
print calc2.y # 35
• Data attributes are
variables owned by a
particular instance
• Each instance has its own
value for data attributes
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Data attributes (2)
class Calculator():
counter = 0
def __init__(self, x=0, y=0):
self.x = x
self.y = y
calc = Calculator(10, 20)
calc.z = calc.x + calc.y
print calc.z
print calc.__dict__
Output:
30
{'y': 20, 'x': 10, 'z': 30}
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• In Python classes you don’t
have a restriction of
declaring all data
attributes before hand,
you can create data
attributes at runtime
anywhere
• calc.z is an attribute
which is defined at
runtime outside the class
definition

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Class attributes
class Calculator():
counter = 0
def __init__(self, x=0, y=0):
self.x = x
self.y = y
def add(self):
self.__class__.counter += 1
return self.x + self.y
calc = Calculator(10, 20)
print calc.add()
print calc.counter # 1
calc2 = Calculator(30, 40)
print calc2.add()
print calc2.counter # 2
• Access the class
attribute using
self.__class__.count
er
• Class attributes are
shared among all
instances
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Class attributes (2)
 Class attributes are defined within a class definition
and outside of any method
 Because all instances of a class share one copy of a
class attribute, when any instance changes it, the
value is changed for all instances
self.__class__.attribute_name
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Data attributes
 Variable owned by a
particular instance
 Each instance has its own
value for it
 These are the most
common kind of attribute
Class attributes
 Owned by the class as a
whole
 All class instances share the
same value for it
 Good for
• Class-wide constants
• Building counter of how
many instances of the
class have been made
Data attributes vs Class attributes
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Inheritance
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Inheritance
class Shape(object):
def name(self, shape):
print "Shape: %s" % shape
class Square(Shape):
def area(self, side):
return side**2
Shape is the
parent class
Square is the
child class inherits
Shape
class Parent(object):
pass
class Child(Parent):
pass
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Inheritance (2)
class Shape(object):
def name(self, shape):
print "Shape: %s" % shape
class Square(Shape):
def area(self, side):
return side**2
s = Square()
s.name("square")
print s.area(2)
Output:
Shape: square
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Child class Square has access to
Parent classes methods and
attributes
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Method overriding
class Shape(object):
def name(self, shape):
print "Shape: %s" % shape
class Square(Shape):
def name(self, shape):
print "Child class Shape %s" % shape
def area(self, side):
return side**2
s = Square()
s.name("square")
print s.area(2)
Output:
Child class Shape square
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Calling the parent method
class Shape(object):
def name(self, shape):
print "Shape: %s" % shape
class Square(Shape):
def name(self, shape):
super(Square, self).name(shape)
print "Child class Shape %s" % shape
def area(self, side):
return side**2
s = Square()
s.name("square")
Use super keyword to call parent class method
super(ChildClass, self).method(args)
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Class & Static Methods
class Calculator(object):
counter = 0
def __init__(self, x=0, y=0):
...
def add(self):
...
@classmethod
def update_counter(cls):
cls.counter += 1
@staticmethod
def show_help():
print 'This calculator can
add'
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Resources
 http://www.diveintopython.net/object_o
riented_framework/defining_classes.html
 http://docs.python.org/2/tutorial/classes
.html
 http://docs.python.org/2/library/function
s.html#super
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